Ms May revealed her type 1 diabetes diagnosis in 2013, but it is unclear for how long she has been using the device.

Diabetes Australia spokeswoman Renza Scibilia said the FreeStyle Libre flash glucose monitoring device was "really useful" for many people with diabetes.

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Theresa May was spotted wearing a diabetes patch when she met Donald Trump for a black-tie dinner in London.Credit:AP

"People with diabetes need to monitor their glucose levels, and the flash device is a way of doing it that doesn't involve actually needing to prick your fingers frequently," she said.

The device has a small sensor that lasts for two weeks and monitors the fluid just under the surface of the skin. The person wearing it can then scan it with another device or a mobile phone to get their blood sugar level.

"While it's telling you what your glucose levels are at that moment it's also giving you a historical reading as well and showing you what your glucose levels have been for the previous up to eight hours," Ms Scibilia said.

Photographs of Ms May with the US President show her wearing the blood glucose monitoring device.Credit:AP

Ms Scibilia, who has type 1 diabetes herself, said she has tried the device and found it "extremely convenient" to use, but added that it was not perfect for everybody.

"It’s all about choice, and there are thankfully many options that work for many people, but for Theresa May it seems that this is her choice," she said.

The device has been available in Australia for two years. Not yet subsidised, each sensor costs $92.50.

A finger pricking test is another way of checking blood sugar levels.Credit:Matthew Bouwmeester

Despite the cost, Ms Scibilia said the uptake in Australia had been "quite significant".

"A number of people have found that for them it is certainly worth spending that money," she said.

Type 1 diabetes not just diagnosed in children

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While the illness used to be known as juvenile diabetes, Ms Scibilia said diagnoses later in life - such as Ms May's - were common.

"There are studies out to show that over 50 per cent of people [with type 1 diabetes] are diagnosed as adults," she said.

Type 1 diabetes occurs when there is a complete lack of insulin in the body, meaning people with the condition have to give it to themselves.

"So for people with type 1, it's a constant balance between putting in the right amount of insulin, eating the right amount of carbohydrates, and monitoring and keeping an eye on all of those things to make sure that our glucose levels don't go too high and that they don't go too low," Ms Scibilia said.

Catching the early signs of type 1 diabetes

It also happens to be National Diabetes Week, and Ms Scibilia said this year the organisation wants people to learn the early signs of type 1 diabetes.

"Each year, 640 patients are winding up in emergency really, really unwell because the signs of type 1 diabetes have been missed," she said.

"It's toilet, so needing to wee more frequently than usual; being really tired - just exhausted; thirsty, and that's having a thirst that almost feels unquenchable; and being thinner, so people losing weight unexpectedly," Ms Scibilia said.

If people notice the signs in themselves or their children, she said they should go straight to their GP and ask for the test.

"It's just a matter of weeing into a little cup and checking glucose levels."